This invention relates to a process for the formation of a concave micro-relief in a substrate and use of this process for making different types of components or optical systems.
The invention is used for applications in micro-optics and particularly for the manufacture of concave mirrors, and also plane-concave, convex-convex or concave-concave type lenses. These components may be in individual form or may be integrated in optical systems. Micro-relief is also useful in the biotechnology field.
Note that the process according to the invention may also be used to make classical, cylindrical or spherical components with a given constant radius of curvature, or non-spherical components with a variable radius of curvature.
One particular application of the invention is the manufacture of a cavity and/or a mirror with a concave cavity for a microlaser with an unstable cavity, or for the production of a microdish that can be used in biotechnology equipment.
For the purposes of this invention, the terms convex and concave are used to qualify the type of curved surface of an optical component, a layer of material or a substrate. These terms are applicable to the curved surface as seen from the outside of the said component, layer or substrate.
At the present time, there are several known techniques for manufacturing curved surfaces. Most of these techniques have been developed for the fabrication of spherical or cylindrical lenses.
FIGS. 1 to 3 in the appendix diagrammatically show the main steps of a process designed to apply a convex relief to the surface of a substrate.
A first step illustrated in FIG. 1 comprises the definition of a resin embossment 10 on a substrate 12. In this case the embossment is a disk. For example, the embossment 10 may be formed by the deposition of a photosensitive resin layer and by a photolithography treatment to eliminate the layer away from the embossment. The embossment 10 is centered on the part of the substrate on which the surface is to be curved.
A second step illustrated in FIG. 2 consists of heating the resin embossment to make it melt. During this heat treatment, surface tensions will vary the shape of the resin embossment to change it to the form of a spherical drop (convex).
As the heat treatment is continued, the resin becomes cross-linked and solidifies while keeping its spherical shape.
A third step consists of transferring the shape of the resin drop into the substrate. This transfer takes place by applying a vertical anisotropic etching to the substrate and the resin. This etching eliminates a thickness of the substrate, that is thinner when the resin is thicker, at all points on the surface of the substrate. The resin is also eliminated as the etching progresses.
After the resin has been completely eliminated, the etching is stopped. This gives a substrate 12 conform with FIG. 3 with a convex relief locally corresponding to the shape of the resin drop (that has disappeared).
The production of concave relief is more complex and more expensive. It requires an additional number of steps in the process.
According to a first technique, a substrate according to the substrate 12 in FIG. 3 can be used as a stamping die to make a concave relief with a complementary shape in another substrate.
In particular, the stamping technique may be used to form the complementary concave relief in a layer of photosensitive resin. This resin layer may then be used for transfer by etching of the concave shape in a substrate, in accordance with the third step in the process described above.
Obviously in this case, the melting heat treatment is not performed on the resin, to avoid eliminating the relief formed by stamping.
Examples of this technique are described in documents (1) and (2), for which the references are given at the end of this description.
Other techniques for producing a concave relief have been developed in order to avoid the need to make a stamping die in advance and the corresponding costs.
These techniques also make use of a resin layer, and more particularly a photosensitive resin layer that can be formed according to photolithographic processes. A concave relief is applied to the resin by insolation for which the intensity is controlled locally.
The insolation intensity may be controlled (as shown in document (3)), using a lithography mask. The mask is used to create different xe2x80x9cgray shadesxe2x80x9d and thus modulate the depth of the insolated resin.
This technique is advantageous in that it enables the development and structuring of a resin layer with a given relief in a single insolation cycle. However, lithography machines for embossment are expensive. The development and use of machines for the above process are also long and complex operations.
Insolation may also be controlled by controlling a laser beam or an electron beam used as the insolation source.
According to one possibility, the power of the source may be varied to correspond with the region of the insolated resin layer.
According to another possibility, the power of the source can also be kept constant while scanning the resin layer. In this case, the insolation is controlled by adjusting the speed and the relative displacement directions between the insolation source and the resin layer.
These techniques make use of opto-mechanical treatment benches that are also expensive and complex. They also introduce difficulties related to making joints at the center of patterns with a symmetry of revolution. These difficulties can reduce the quality of the relief obtained and therefore make the technique unsuitable for the manufacture of some optical components.
For example, an illustration of the techniques involving control of the insolation source is given in documents (4) and (5). The references of these documents, and the references of other documents mentioned, are given at the end of the description.
The resin layer may be scanned by an insolation beam combined with the use of a mask, as described in document (6). Therefore the technique described in this document is similar to the techniques described above and has more or less the same disadvantages.
Document (7) describes a method for making concave relief that does not make use of an intermediate layer of photosensitive resin that will define the shape of the relief.
The substrate in which it is required to form the relief is etched directly using a reactive ionic etching process. The etching anisotropy is controlled by adjusting the dimensions of an etching mask and the reactive etching parameters.
This technique is apparently advantageous, and has a scope limited to materials that may be etched using a reactive plasma. Furthermore, it cannot be used to obtain a good optical surface quality with a wide range of radii of curvature, or a good manufacturing efficiency.
A better manufacturing efficiency may be obtained by collective treatment of optical component preforms. According to this method, the preforms are embedded in a coating material and are then abraded and polished with the coating material. The surface of components may be made convex by selecting a coating material with a lower resistance to abrasion than the resistance of the preform materials, or it may be made concave if the abrasion resistance of the coating material is higher.
For example, this abrasion treatment may be applied to the production of plane-convex type microlaser cavities.
These cavities, also called unstable cavities, are used to increase the size of the beam and therefore the output power of the microlasers on which they are used. The concave radius of curvature of the surface of the microcavities is within a range from a few millimeters to a few hundreds of millimeter, for diameters ranging from a few tens of micrometers to a few hundred micrometers.
Documents (8) and (9) provide illustrations of these cavities.
As described above, the technique for the formation of concave (or convex) relief by abrasion is suitable for the collective and economic manufacture of optical components such as microlaser cavities. However, they cannot always give a required value of the radius of curvature or a very good optical quality.
Furthermore, the described process requires many preliminary preparation steps that cannot always be done collectively.
The purpose of the invention is to propose a process that enables the formation of a concave relief in a substrate that can be used for making optical components or optical systems, and that does not have the difficulties or limitations of the techniques mentioned above.
One purpose in particular is to propose such a process that is economic and adapted to processing for the collective production of a large number of components.
Another purpose is to propose such a process that uses a small number of operating steps and that enables precise control of the resulting radii of curvature and relief.
Another purpose is to propose such a process that can be used to make optical layers or components with a surface condition with a good optical quality and that can be used to make spherical or non-spherical relief.
Another purpose of the invention is to propose processes for making plane-concave, concave-concave or convex-convex type lenses, and optical systems comprising one or several of these lenses.
More precisely, the objective of the invention in order to achieve these purposes is a process for making at least a concave relief in a substrate comprising the following steps:
formation of at least one embossment on the substrate, made of a material subject to creep, the embossment of material subject to creep having a shape factor defined by the ratio of the average height h of the embossment to the dimensions of a contact area s between the embossment and the substrate,
heating of the material subject to creep to a sufficiently high temperature to cause creep of the said material, and
etching of the substrate and the crept material to form a relief on the substrate.
For example, the dimension of the area s used is usually the largest dimension or the diameter in the case of an area s with a symmetry of revolution.
The material subject to creep may advantageously be a photosensitive resin or a meltable material such as a metal alloy with a low melting point.
According to the invention, at least one of the parameters chosen among the shape factor and the heating temperature is selected to apply a concave relief to the embossment and the crept material is solidified in a state in which it has the said concave relief. In the case of a resin, this solidification corresponds to cross-linking.
In the following text, the manufacture of a single relief is described starting from a single embossment. However, the invention can be used by forming one or several embossments on one or several substrates, and then collectively processing the substrates and/or embossments.
The invention is based on a property of materials subject to creep such as resins, according to which these materials do not immediately change to a final stable spherical shape, but gradually creep towards this shape passing through intermediate shapes with concave parts.
The development of the embossment made of material subject to creep into the final spherical shape is faster when the quantity of material that forms the embossment is larger and when the heating temperature is higher.
Furthermore, the inventors have observed that when the height of the embossment material compared with the considered dimension of its area is small, in other words when a shape factor of the embossment is less than a given threshold value, the embossment cannot change towards the final spherical form, but stabilizes as a shape (intermediate) with a concave region.
This particular feature may be used in a first possible embodiment of the invention. An embossment with a shape factor less than a threshold value can be made, starting from which creep can take place towards a final convex stable shape. The creep heat treatment is followed by solidification of the crept material.
This solidification may for example be achieved by quenching in the case of a meltable material, or by heating in the case of a resin (heating induces cross-linking of the resin).
The crept material is solidified to consolidate it and thus fix the resulting concave shape to prevent it from being affected by subsequent manipulation of the substrate.
Considering that the embossment of material subject to creep, for example cylindrical or in the shape of a disk, has an approximately circular contact area with the substrate, the shape factor may also be defined by the height h and the diameter D, for example the ratio h/D.
In this case, in order to enable the embossment to stabilize in its concave shape, the shape factor h/D is preferably selected to be less than a threshold value as follows:
h/D less than 0.015
Note that the threshold value indicated above corresponds to resins that are usually used in known photolithographic techniques and that are perfectly usable within the framework of the invention.
According to another possible embodiment of the invention when a resin embossment is used, it is also possible to make the embossment using a shape factor greater than or equal to the value of the threshold starting from which creep would be possible to a stable convex shape. In this case, heating is applied to a sufficiently high temperature to cause cross-linking of the resin before the stable convex shape is reached.
In this case, cross-linking corresponds to solidification and its main purpose is to fix the variation of the shape of the resin while creep is taking place.
The radius of curvature of the concave shape reached may be adjusted by varying the cross-linking time, to shorten it or to extend it. This may take place by controlling the heating temperature. For frequently used resins, heating may usually be done at a temperature of more than 120xc2x0 C.
After creep of the embossment has taken place, the substrate and the crept material may be etched to form a shape (concave) that depends on the shape (concave) of the crept material. It is also said that the shape of the crept material is xe2x80x9ctransferredxe2x80x9d into the substrate.
Etching of the substrate and the crept material may be characterized by its selectivity. The selectivity is defined as being the ratio between an etching rate of the material from which the substrate is made, denoted Vs, and an etching rate of the crept material, denoted Vr. The rates Vs and Vr depend on the materials, and also on the etching agents used. Let the selectivity be S, then:
S=Vs/Vr
The selectivity of etching induces a modification to the radius of curvature between the radius of curvature of the concave shape of the crept material and the radius of curvature of the shape transferred into the substrate.
Considering that the concave shape of the crept material and the concave shape transferred into the substrate are approximately spherical, and denoting their radii of curvature as Rr and Rs respectively, the following relation is satisfied:
Rr/Rs=Vs/Vr=S
The result is that the radius of curvature Rs that is applied to the substrate may be adjusted at will either by modifying the radius of curvature of the crept material, or particularly by modifying the selectivity of the etching.
The selectivity of the etching may be kept constant throughout the etching step. In this case, the spherical character of the shape of the crept material is kept during the transfer.
However, it will also be possible to continuously or discontinuously vary the selectivity of etching. This may be done for example by varying the composition of the etching agents used. For example, for plasma etching, the concentration or the flow of the different reagent gases used (CHF3/O2 or SF6/O2) may be varied. The variation of the selectivity enables a spherical shape to be transferred into the substrate starting from an approximately spherical shape of the crept material.
According to another aspect of the invention, it is possible to interrupt etching before or after the complete disappearance of the crept material.
When etching is interrupted before complete disappearance of the crept material, the remaining crept material may be eliminated, for example dissolved, to leave a portion of the substrate in its initial shape around the transferred shape. In particular, when the surface of the substrate is initially plane, this process can be used to surround the concave shape with a flat.
The process mentioned above may be used to make different types of optical components, particularly such as concave mirrors, for example for laser cavities or lenses or plane-concave relief, plane-convex lenses or concave-concave or convex-convex lenses. More complex systems can also be produced.
Mirrors can easily be obtained, for example by metallizing the concave relief formed in a substrate in accordance with the invention.
One or several embossments can be formed on one of the main faces of a substrate with parallel faces, to make one or several plane-concave relief patterns. The steps in the process described are then applied. The substrate is then preferably made of a material with a refraction index and transparency adapted to the production of a lens or other optical component such as a laser cavity.
The production of a plane-convex type lens may include the following steps:
a) formation of a concave relief in the substrate in accordance with the process described above;
b) conform deposition on the substrate of a layer of optical material matching the shape of the relief;
c) flattening of an exposed face of the said layer to make it approximately plane.
An optical material is a material adapted to the production of a lens or an optical component, in other words a material with an index and transparency required for the envisaged application.
The process for manufacturing a plane-convex lens may optionally also include the deposition of a sacrificial material on the substrate before step b), and elimination of the layer of sacrificial material after step b) or step c) to expose the layer of optical material.
The production of a concave-concave lens may include the following steps:
a) formation of an embossment of material subject to creep on a substrate;
b) creep of the said material until a stable creep state is reached in which the crept material has a convex shape;
c) etching of the substrate and the crept material to apply a convex relief to the substrate;
d) deposition of a layer of optical material on the substrate matching the convex relief of the substrate;
e) flattening of an exposed surface of the layer of optical material;
f) formation of concave relief in the layer of optical material, in accordance with the process according to the invention described above, the concave relief being formed in a concave region of the layer of optical material corresponding to the convex relief of the substrate.
According to one particular embodiment of the lens manufacturing process, the process may comprise deposition of an overlayer on the optical material layer after step f), and planarization of the said overlayer.
In the same way as when manufacturing a plane-concave lens, it is possible that the lens can be separated from the substrate.
In this case, the process includes the deposition of a layer of sacrificial material before step d), and elimination of the layer of sacrificial material after step f) to expose the layer of optical material.
Note that in this process, the type of material used for the substrate may be selected freely without taking account of optical requirements such as the index or transparency.
Production of a convex-convex lens may comprise the following steps:
a) formation of a concave relief in a substrate according to the process according to the invention described above;
b) deposition of a layer of optical material matching the said concave relief, on the substrate;
c) formation of an embossment of material subject to creep on the layer of optical material, approximately facing the said concave relief;
d) creep of the embossment until a stable creep state is reached in which the shape of the crept material is convex;
e) etching of the crept material and the layer of optical material to apply the convex shape in the layer of optical material.
The process may also comprise the formation of a layer of sacrificial material on the substrate before step b), and elimination of this layer after step e), to expose the optical layer and the lens.
Other characteristics and advantages of the invention will be more easily understood after reading the following description with reference to the figures in the attached drawings. This description is given for illustration purposes only and is in no way restrictive.
The following description refers to examples in which the material used subject to creep is a resin, for simplification reasons.